Steps

Output for production

Serial port

The payload(if you use Seabios from coreboot's "make menuconfig" it's already done for you)

The bootloader like grub

The kernel(like the linux kernel)

The init system(systemd, sysvinit etc...)

dbgp

if you don't have a serial port you could enable the USB debug port in coreboot and grub. However in my GNU/Linux disribution(Parabola, based on arch) they didn't enable it for the linux(-libre) kernel.